COBB, CHESTER RAY. Estimating nitrogen efficiency of swine lagoon liquid applied to field crops using continuously variable irrigation. (Under the direction of Robert L. Mikkelsen). Application of anaerobic swine (Sus scrofa domesticus) lagoon liquid onto cropland by irrigation is a common method of waste disposal and treatment. Currently, the application rate of swine lagoon liquid is based on the N concentration of the lagoon liquid and the N required by the receiver crop to obtain a realistic yield. In North Carolina, only 50% of the total N in the swine lagoon liquid applied by irrigation is considered available for plant use during the first year after application. Uncertainty exists as to whether this coefficient accurately predicts the amount of plant-available N. Therefore, research was conducted in the Coastal Plain of North Carolina to determine the efficiency of N uptake by corn (Zea mays L.) and soybean (Glycine max Merrill) receiving swine lagoon liquid through irrigation. The line-source sprinkler irrigation method was used to provide a continuous variable N rate, ranging from 0 to 290 kg N ha-1, across the field during 1999 and 2000. Ammonia volatilization losses ranged from 6 to 22% during irrigation. Crop yield and grain N recovered were affected more by the amount of liquid than N applied in 1999. Nitrogen recovered in grain in 1999 was < 15% for both corn and soybean at 168 kg N ha-1 of either swine lagoon liquid or NH4NO3. In 2000 at the 168 kg N ha-1 rate, grain N removal by corn, nonnodulating soybean, and nodulating soybean was 28, 25, and 39% from swine lagoon liquid and 45, 31, and 56% from NH4NO3. Based on yields and grain N removed by corn and nonnodulating soybean in 2000, N from applied swine lagoon liquid, accounting for N losses during irrigation, was about 70% as effective as NH4NO3. Symbiotic N2 fixation by the soybean was reduced by 60% when applied N reached 175 kg N ha-1 for both NH4NO3 and swine lagoon liquid. While nodulating soybean removed more grain N than did either corn or nonnodulating soybean in 2000, soil inorganic N concentrations at the end of the growing season were higher for the nodulating soybean. Therefore, it is not conclusive if soybean would be a better receiver crop than corn for swine lagoon liquid. Based on the results of this study, using the 50% available N coefficient of the lagoon liquid comes close to predicting plant-available N when N losses during irrigation are around 25%. Nitrogen losses during irrigation can significantly affect plantavailable N when applied N is based on the N concentrations of the lagoon liquid.